1. Technical Field
The invention relates to a measuring apparatus and a measuring method for measuring an interaction between a measurement-target sample substance and a bioactive substance.
2. Related Art
Conventionally, a technique for selecting a sample substance containing functional groups bonded to reactive groups contained in a bioactive substance from among various candidates is known. Examples of the bioactive substance include specific proteins. The sample substance, such as a nucleic acid or a derivative thereof, combined with such a specific bioactive substance is expected to be used mainly in the field of medicine.
As a method of selecting a sample substance to be combined with a bioactive substance, various methods of evaluating an interaction between the bioactive substance and the sample substance have been used. An evaluation method using near field light because of no need of label and high sensitivity is known as an evaluation method. Examples of the method of measuring the interaction using the near field light include a method using surface plasmon resonance as disclosed in, for example, Japanese Patent (JP) No. 2758904 and Biacore Application Training.
Generally, if the interaction between the bioactive substance and the sample substance is to be measured using the surface plasmon resonance, then a sample solution obtained by dissolving the measurement-target sample substance into an organic solvent is supplied to a measurement region arranged on one surface of a prism and made of a bioactive substance-immobilized film, a laser beam is irradiated onto the bioactive substance-immobilized film at various angles so as to obtain a total reflection condition on an interface between the bioactive substance-immobilized film and the bioactive substance, and the interaction between the bioactive substance and the sample substance is measured based on a result of detecting a refraction index of the light totally reflecting on this interface.
Furthermore, to acquire more accurate information, the following method is sometimes employed. A measurement region in which a bioactive substance is immobilized and a reference region in which the bioactive substance is unimmobilized are provided. By correcting signal information acquired from the measurement region using signal information acquired from the reference region, the interaction between the sample substance and the bioactive substance is shown.
According to a technique disclosed in, for example, the Biacore Application Training, a plurality of kinds of concentration control solutions different in organic solvent concentration are prepared. Refraction indexes are measured when the concentration control solutions having different organic solvent concentrations are supplied to each of the reference region and the measurement region. The relationship between the refraction index measurement result and the organic solvent concentration in each of the reference region and the measurement region is obtained in advance. In an ideal state, the relationship between the refraction index measurement result and the organic solvent concentration is a primary linear relationship (hereinafter, referred to as “ideal linear line”). In the measurement region, the ideal line differs from the reference region in inclination because of immobilization of the bioactive substance onto the bioactive substance-immobilized film. Due to this, the refraction index measurement result in the measurement region and that in the reference region obtained by supplying the sample solutions to each of the measurement region and the reference region are corrected based on the organic solvent concentrations of the respective sample solutions and the primary linear relationship obtained in advance, thereby measuring the interaction between the bioactive substance and the sample substance.
However, the relationship between the organic solvent concentration and the refraction index measurement result is not actually the primary linear relationship but has a slight offset (so-called linearity error) with respect to the ideal line. Due to this, if the above-stated method is used, the measurement result indicating that there is actually an interaction between the bioactive substance and the sample substance although there is actually no interaction therebetween may possibly obtained. Furthermore, even if there is an interaction between the bioactive substance and the sample substance, the measurement results indicating a different amount from an actual interaction amount (e.g., a combined amount between the bioactive substance and the sample substance) are sometimes obtained.
To dissolve the sample substance, the above-stated organic solvent is used. It is known that the refraction index of the organic solvent is so high as to influence refraction index information that indicates the interaction between the bioactive substance and the sample substance. Due to this, a concentration fluctuation of the sample solution resulting from a fluctuation in the concentration of the organic solvent in the sample solution due to evaporation or the like is one cause for the linearity error.